1. Field of the Invention
This invention relates to rapid and complete mixing of fluids in a continuous manner by a process that mixes very thin liquid sheets of the different fluids together.
2. Description of Prior Art
The prior art has been concerned for many years with the rapid and complete mixing of liquids so as to reduce segregation of components within the mixture. Less than very rapid and complete dispersion is particularly deleterious in processes utilizing very fast reactions. The term fast reaction implies a reaction that has a time scale that is more rapid or on the same order of the time scale of mixing of the reactants. If the fast reactions are complex, i.e., they involve reactions that are multistep, then product distribution can be adversely affected (see J. Y. Oldshue, Fluid Mixing Technology, McGraw-Hill Publications Co., New York, N.Y., 1983, pp. 222-229). Segregation that occurs due to inhomogeneities within the mixture on the molecular scale can change the product distribution from that calculated assuming perfect and complete mixing before the reactions, begin. Particularly with multiple reactions failure to pay attention to segregation within the mixture can cause wastage of raw materials in producing undesired substances, difficulties in scale-up, and an increased load on the separation plant (see J. R. Bourne, F. Kozicki, and P. Rys, "Mixing and Fast Chemical Reaction", Chem. Eng. Sci., 36 (10), pp. 1643-1663, 1981).
In general, liquid phase reactions occuring in viscous media, such as polymerization and biochemical reactions, are particularly subject to the influence of segregation. A recent symposium "Rapid Mixing and Sampling Techniques in Biochemistry" explained the problems of characterizing biochemical reactions that proceeded more rapidly than the time scale of the initial mixing of the reactants (see Chance, B., et. al. (eds.), Rapid Mixing and Sampling Techniques in Biochemistry, Academic Press, New York, N.Y., 1964).
Prior art for rapid mixing generally uses jets of liquid that impinge against one another or tangentially mounted feed tubes that mix the fluids in a swirl cup. An example of such a mixing device is given in U.S. Pat. No. 4,239,732, granted Dec. 16, 1980 to F. W. Schneider. These types of mixers can give fairly complete mixing of very small amounts of liquids in times as low as milliseconds for low viscosity fluids. However, these streams are relatively thick and this limits the speed with which solutions can be mixed. It has long been known that if two or more liquids can be made as thin as possible before they are mixed, then rapid and complete mixing is virtually assured (see p. 49-53 in Chance, B. et. al., supra).
To this end a Russian scientist, Yu B. Kletenik in the Russian Journal of Physical Chemistry, Vol. 37(5), p. 638 (May, 1963) has devised a mixing device that mixes thin liquid layers together. It does this by flowing two liquids between very narrow parallel plates similar to a triple decker sandwich. Between the first two plates the first fluid flows and between the second and third plate the second fluid flows. The liquids are accelerated to high velocities (two meters/second or higher), so that they flow separately through the parallel plates, and are mixed once they flow beyond the end of the plates and into free space. With this system Kletenick claims to have obtained mixing times on the order of 90 to 100 microseconds for low viscosity liquid sheets of 200 microns thickness.
Although Kletenick claims that his device provides fast mixing, it suffers from a number of drawbacks.
(1) Since the device requires flow between two parallel flat plates with a very narrow gap and of significant length, the pressure drop is relatively high. Any attempt to further decrease the size of the thin film produced further increases the pressure drop. This is particularly severe if the reactants are viscous.
(2) The width of the plates themselves must also be very thin (100 microns), otherwise the fluids will completely miss each other once they flow out the end of the narrow gaps. Such a device is not only difficult to construct but is also very delicate, rendering it unsuitable for industrial use. Applications where the fluids must be injected at only moderately high pressures are not feasible.
(3) The device is limited to a gap of about 0.1 mm between plates which limits the thinness of the sheets formed to 0.2 mm or 200 microns as per Kletenick's analysis of how his mixing device works.
(4) Because of the very narrow plate gaps plugging is a potential problem in systems containing suspended solids.
(5) Kletenick's device is impractical at usual industrial flowrates of liters per minute and higher.